patch

The patch takes two arguments, the first is a DOM element (you can get it using emscripten::val::global) or a vnode representing the current view. The second is a vnode representing the new, updated view. If patch succedeed, the new vnode (the second parameter) is returned.

If a DOM element is passed, newVnode will be turned into a DOM node, and the passed element will be replaced by the created DOM node. If an oldVnode is passed, asm-dom will efficiently modify it to match the description in the new vnode.

If unsafePatch in init is equal to false, any old vnode passed must be the resulting vnode from the previous call to patch. Otherwise, no operation is performed and NULL is returned.

VNode* oldVnode = h("span", std::string("old node"));
VNode* newVnode = h("span", std::string("new node"));

patch(
    emscripten::val::global("document").call<emscripten::val>("getElementById", emscripten::val("root")),
    oldVnode
);
patch(oldVnode, newVnode);

// with unsafePatch = false
VNode* vnode = h("div");
patch(oldVnode, vnode); // returns NULL, found oldVnode, expected newVnode

With unsafePatch = true you can implement some interesting mechanisms, for example you can do something like this:

VNode* oldText = h("span", std::string("old text"));
VNode* vnode = h("div", Children {
  h("span", std::string("this is a text")),
  oldText
});

patch(
    emscripten::val::global("document").call<emscripten::val>("getElementById", emscripten::val("root")),
    vnode
);

VNode* newText = h(std::string("span"), "new text");
// patch only the child
patch(oldText, newText);